Image forming apparatus

ABSTRACT

An image forming apparatus includes: a sheet transport mechanism including: a sheet feeding unit; an attitude correction unit that corrects an attitude of a sheet; and a drive source; a first actuator that operates upon receipt of supply of power from a power source, when a sheet feeding command is issued, and transmits power of the drive source to the sheet feeding unit to commence driving of the sheet feeding unit; a second actuator that operates upon receipt of power from the power source and controls the attitude correction unit into an attitude correctable state, when a transported sheet has approached the attitude correction unit; and a control unit that determines an operation start timing of the first actuator, operation of the first actuator, which transports the next sheet after transport of one sheet, starting after operation of the second actuator for the one sheet has been completed.

CROSS-REFERENCE TO THE RELATED APPLICATION(S)

This application is based upon and claims a priority from prior Japanese Patent Application No. 2005-182214 filed on Jun. 22, 2005, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

Aspects of the present invention relate to an image forming apparatus.

BACKGROUND

A conventional image forming apparatus transports a sheet placed on a sheet feeding cassette to a transfer position (a nip position between a photosensitive drum and a transfer roller) by a plurality of rollers, thus forming a desired image on the sheet. As disclosed in JP-A-2005-114754, a sheet feeding roller, registration rollers, a transfer roller, a heating roller, a pressurization roller and a sheet discharge roller are sequentially arranged along a sheet transport path in the image forming apparatus. When transport of the sheet is started by a driving action of the sheet feeding roller, the transported sheet is delivered first to the registration rollers. The registration rollers correct the attitude of the transported sheet. The registration rollers normally rotate. However, when the sheet approaches the registration rollers, the registration rollers halt temporarily. When a leading end of the sheet comes into contact with the stationary registration rollers, the attitude of the sheet is corrected.

Subsequently, by means of a driving action of the registration rollers, the sheet is delivered to the transfer roller. In association with the sheet passing by the transfer roller, a toner image is formed on the sheet. In association with the sheet passing by the heating/pressurization roller, the toner image is thermally fixed and subsequently discharged.

The respective rollers are connected to the motor, which acts as a drive source, by way of a relay gear (a power transmission mechanism), and receive power from the motor. The sheet feeding roller and the registration rollers are each provided with a clutch mechanism (of electromagnetic type having a solenoid switch) that intermittently transmits power to the sheet feeding roller or the registration roller. The reason why the sheet feeding roller is provided with a clutch mechanism is that sheets must be fed intermittently one at a time. When an excitation current is supplied to the coil of the solenoid switch, the power transmission mechanism is connected to the solenoid switch, to thus rotate the sheet feeding roller.

In the meantime, as mentioned previously, the reason why the registration rollers are provided with the clutch mechanism is for temporarily stopping rotation of the roller in association with an approach of the sheet. When an excitation current is supplied to the coil of the solenoid switch for the registration rollers, the power transmission mechanism is disconnected, whereupon rotation of the rollers stops.

SUMMARY

As mentioned above, when solenoid switches are used for the registration rollers and the sheet feeding roller respectively, there is a potential risk of a coincidence arising between the timings at which an electric current is supplied to coils of both solenoid switches. Specifically, in the image forming apparatus, a transport interval between sheets is determined in advance, and an electric current is supplied to the solenoid for the sheet feeding roller in synchronism with the transport interval. However, the timing when the transported sheet approaches the registration rollers may coincide with the timing when the next sheet is transported (an area enclosed by a dashed line in FIG. 10A).

Exciting the coil of the solenoid switch usually requires a comparatively large electric current. Hence, when the timings of supply of an electric current to the coils of both solenoid switches coincide with each other, there may arise a case where a load exceeds the performance of the power source, thereby hindering stable supply of power. In such a case, in order to offset the timings of supply of an electric current to the coils of both solenoid switches, it is better to control the timing when the sheet, whose transport has been started, approaches the registration rollers. To implement control of approach timing, providing a mechanism for temporarily stopping transport of the sheet between the sheet feeding roller and the registration rollers is conceivable. However, this leads to an increase in the number of components, which increases the cost.

In addition, in order to offset timings of an electric current supply to the coils of both solenoid switches, setting custom layouts for the respective registration rollers is conceivable. Specifically, the layout of the registration rollers is determined beforehand such that the timing of an approach of the transported sheet to the registration rollers does not coincide with the timing of transport of the next sheet.

In recent years, some image forming apparatus use sheet feeding cassettes C1, C2 arranged in layers, pursuant to the user's request (see FIG. 11). In this case, transporting a sheet from the lower cassette C2 results in an increase in a transport length from a transport start position to the registration rollers, when compared with a case where a sheet is transported from the upper cassette C1. As shown in FIG. 10B, when a sheet is fed from the lower cassette C2, the timing of an approach of a sheet to the registration rollers becomes delayed as compared with a case where the sheet is fed from the upper cassette (the area enclosed by a dashed line in FIG. 10B).

For instance, in a case where the sheet is fed from the cassette C1, even when the timings of supply of an electric current to the coils of the solenoid switches are offset from each other, there will arise a case where the timings of supply of an electric current to the coils of both solenoid switches coincide with each other if a sheet is fed from the cassette C2.

Aspects of the present invention provide an image forming apparatus that can avoid simultaneous supply of an electric current to both solenoid switches, without increasing the number of components.

In order to achieve the above object, according to an aspect of the invention, there is provided an image forming apparatus including: a sheet transport mechanism including: a sheet feeding unit that feeds a sheet from a sheet feeding cassette; a attitude correction unit that corrects an attitude of the sheet fed from the sheet feeding unit; and a drive source that drives the sheet feeding unit and the attitude correction unit; a first actuator that operates upon receipt of supply of power from a power source, when a sheet feeding command is issued, and transmits power of the drive source to the sheet feeding unit to commence driving of the sheet feeding unit; a second actuator that operates upon receipt of power from the power source and controls the attitude correction unit into an attitude correctable state, when a transported sheet has approached the attitude correction unit; and a control unit that determines an operation start timing of the first actuator, operation of the first actuator, which transports the next sheet after transport of one sheet, starting after operation of the second actuator for the one sheet has been completed.

According to the aspect of the invention, the first and second actuators are prevented from being simultaneously actuated; namely, the actuators are prevented from being simultaneously supplied with a drive current from the power source. As a result, a load exceeding the performance of the power source is avoided. Hence, power is stably supplied from the power source, thereby rendering operation of the image forming apparatus stable.

Operation timings of the actuators is controlled by the control unit. Hence, devices dedicated for the control unit are minimized. Accordingly, there is no risk of a significant increase in the number of components.

According to another aspect of the invention, there is provided an image forming apparatus including: a sheet transport mechanism including: a sheet feeding unit that feeds a sheet from a sheet feeding cassette; an attitude correction unit that corrects an attitude of the sheet fed from the sheet feeding unit; and a drive source that drives the sheet feeding unit and the attitude correction unit; a first actuator that operates upon receipt of supply of power from a power source every time a sheet feeding command is issued and transmits power of the drive source to the sheet feeding unit to commence driving of the sheet feeding unit; a second actuator that operates upon receipt of power from the power source, when a transported sheet has approached the attitude correction unit, to control the attitude correction unit into an attitude correctable state; and an operation interval determination unit that determining an operation interval time of the first actuator such that the number of operations of the first actuator per unit time does not exceed a preset number; and a control unit that determines operation start timing of the first actuator, operation of the first actuator for the next sheet starting after the operation interval time has elapsed since the start of operation of the first actuator for one sheet.

According to the aspect of the invention, the current supply time during which a drive current is supplied to the first actuator per unit time is maintained at an appropriate value. Consequently, heating of the first actuator, which would otherwise be caused when an electric current exceeding an appropriate amount of electric current flows into the first actuator, can be prevented, whereby operation of the image forming apparatus becomes stable.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention will be more fully apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a side cross sectional view of a laser printer;

FIG. 2 is a cross sectional view showing that an MP tray is opened;

FIG. 3 is a view showing a power transmission mechanism;

FIG. 4 is a partially enlarged view of sheet transport paths;

FIG. 5 is a block diagram showing an electrical configuration of the laser printer;

FIG. 6 is a flowchart showing procedures by which an ASIC determines a sheet transfer timing;

FIG. 7 is a timing chart showing a case where a sheet has been transported from a sheet feeding cassette C1;

FIG. 8 is a view showing the configuration of memory;

FIG. 9 is a timing chart acquired when a sheet has been transported from a sheet feeding cassette C2;

FIGS. 10A and 10B are views showing current supply timings of a related art example; and

FIG. 11 is a view showing a related art image forming apparatus.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE ASPECTS

One aspect of the present invention will be described by reference to FIGS. 1 through 9.

FIG. 1 is a side cross sectional view of a laser printer according to the aspect.

In a laser printer (an image forming apparatus) 10, a cassette housing section 12 is provided at a bottom portion of a main body casing 11. A sheet feeding cassette C1, where sheets employed as a recording medium are accommodated in layers, is loaded into the cassette housing section 12. This laser printer 10 can be additionally provided with other sheet feeding cassettes. In the present aspect, the laser printer 10 is additionally provided with another sheet feeding cassette; namely, a sheet feeding cassette C2 is disposed beneath the sheet feeding cassette C1. In the following descriptions, the right side in FIG. 1 is taken as a front side of the apparatus.

Sheet transport paths are formed within the main body casing 11. During the course of the sheets fed from the respective sheet feeding cassettes C1, C2 being transported over the sheet transport paths, toner images are formed on the sheets. The thus formed toner images are thermally fixed by a fixing unit 36, whereby desired images are formed (detailed description thereof will be provided later).

The sheet transport paths are routed as designated by dashed lines in FIG. 1. In the case of a sheet transport path L1 for the sheet feeding cassette C1, the sheet transport path is inverted toward a rear side of the laser printer 10 at a front upper section of the sheet feeding cassette C1 about 180°. Then, the sheet is horizontally transported toward the rear side of the laser printer 10. The sheet is then inverted about 180° toward the front of the laser printer 10 at the rear side of the laser printer 10, and the sheet reaches a sheet discharge section 14 provided on an upper wall surface of the main body casing 11.

In the case of a sheet transport path L2 of the sheet feeding cassette C2, a sheet is fed in an obliquely upward direction from the front edge of the sheet feeding cassette C2, and the sheet travels upwardly over the path. After the sheet has ascended by the amount essentially corresponding to the height of the sheet feeding cassette C2, the sheet transport path L2 is merged with the sheet transport path L1 for the sheet feeding cassette C1. Specifically, the sheet transport path L2 for the sheet transported from the sheet feeding cassette C2 is longer than the sheet transport path L1 for the sheet feeding cassette C1 by the amount corresponding to the distance from the start of transport to the merge.

As shown in FIG. 2, in the laser printer 10 of the present aspect, a portion of a front wall 15 rotates about a hinge 16 and is maintained in a horizontal position as illustrated. This pivotal portion corresponds to an MP tray 17, by means of which a sheet can be manually inserted. As illustrated, the sheet on the MP tray 17 is delivered rearward of the laser printer 10 from the MP tray 17 and is subsequently merged with the sheet transport path L1 of the sheet feeding cassette C1. A sheet transport path L3 of the MP tray 17 is shorter than the sheet transport path L1 of the sheet feeding cassette C1. Of all the sheet transport paths L1 to L3, the sheet transport path L3 is the shortest. Thus, the laser printer 10 is provided with the three sheet transport paths L1 to L3 that differ from each other in terms of a transport length.

A mechanism for forming an image on a sheet will now be described briefly.

Various rollers (a sheet transport mechanism) are arranged along the sheet transport path (a description is provided hereinbelow while taking the sheet transport path L1 for the sheet feeding cassette C1 as an example) in order to transport a sheet as shown in FIG. 1. Specifically, a sheet feeding roller (a sheet feeding unit) 21, a relay roller 23, registration rollers (an attitude correction unit) 25, a transfer roller 27, a heating roller 38 and a sheet discharge roller 39 are sequentially arranged. The rotary shafts of the respective rollers are coupled to a rotary shaft M1 of a main motor (a drive source) M by way of a transmission gear G, and the respective rollers rotate upon receipt of the driving force of the main motor M.

FIG. 3 shows a portion of the drive system of the laser printer 10. Reference symbol M in the drawings designates the main motor; M1 designates the rotary shaft of the main motor; 21A designates a rotary shaft of the sheet feeding roller 21 for the sheet feeding cassette C1; 23A designates a rotary shaft of the relay roller 23; 25A designates a rotary shaft of the registration rollers 25; and G1 to G8 designate transmission gears. For example, in the case of the rotary shaft 21A for the sheet feeding roller, driving force of the main motor M is transmitted by way of the transmission gear G1→the transmission gear G2→the transmission gear G3→the transmission gear G4→the transmission gear G5. The group of transmission gears is called a power transmission mechanism in the following descriptions.

The sheet feeding roller 21 is for feeding a sheet placed on the sheet feeding cassette C1 to the relay roller 23. The sheet must be transport one sheet at a time. When a plurality of sheets are sequentially transported, the rotation of the roller must be temporarily stopped during the course of transport of the sheets. The power transmission mechanism of the sheet feeding roller 21 is provided with an electromagnetic clutch mechanism (corresponding to a first actuator of the present invention) 60A (see FIG. 3).

The clutch mechanism 60A is formed chiefly from a transmission gear (i.e., the transmission gear G5 in the present aspect), which has along an outer periphery thereof a lock-receiving section 71; a lock arm 61 for regulating rotation of the transmission gear G5; and a solenoid switch 65A. The lock arm 61 is provided so as to be pivotable about a hinge 62. A latch claw 63 is provided at the extremity of the lock arm 61, and the latch claw 63 engages with the lock receiving section 71 of the transmission gear G5.

A partial notch is formed in each of a large diameter teeth section 73 and a small diameter teeth section 74, which are provided on the transmission gear G5 (FIG. 3 shows only a notch of the large diameter teeth section 73, and this notch is hereinbelow taken as a notch section 75). When the latch claw 63 remains engaged with the lock receiving section 71, settings are made such that the notch section 75 of the large diameter teeth section 73 and the notch section 75 of the small diameter teeth section 74 exactly come to positions where the notch sections 75 engage with other, adjacent transmission gears (a gear provided on the rotary shaft 21A and the transmission gear G4).

By means of these settings, the large diameter teeth section 73 of the transmission gear G5 and the adjacent transmission gear G4 are maintained so as not to mesh with each other, and the small diameter teeth section 74 and the gear section of the rotary shaft 21A are maintained so as not to mesh with each other. Thereby, power transmission between the shaft M1 and the shaft 21A is disconnected (hereinafter described as interrupted); namely, driving (rotation) of the sheet feeding roller 21 is stopped.

It is better to activate the solenoid switch 65A, when the sheet feeding roller 21 is rotated from such a state. The solenoid switch 65A has an advancing-and-receding shaft 66A. The advancing-and-receding shaft 66A can be withdrawn upwardly in the drawing by means of supplying an excitation current to the coil. Thus the entire lock arm 61 pivots in an unlocking direction about the hinge 62, whereupon the latch claw 63 is disengaged from the lock receiving section 71, so that the transmission gear G5 becomes pivotable.

At that time, as a result of the transmission gear G5 receiving impelling force from the impelling unit, the transmission gear G5 rotates through a predetermined angle in the direction of arrow A in the drawing, whereby the notch sections 75 come out of the position where the notch sections 75 mesh with the other, adjacent transmission gears. Thus, the teeth of the respective gears mesh with each other, so that a power transmittable state (hereinafter called a “connected state”) is achieved.

The sheet feeding cassette C2 is provided with the custom designed sheet feeding roller 41, and the MP tray 17 is provided with a sheet feeding roller 45. Moreover, the sheet feeding roller 41 is provided with a custom designed electromagnetic clutch mechanism and a solenoid switch 65C, and the sheet feeding roller 45 is provided with a custom designed electromagnetic clutch mechanism and a solenoid switch 65D (see FIG. 5).

The relay roller 23 is for relaying the sheet fed by the sheet feeding roller 21 so as to reach the registration rollers 25, and the relay roller 23 is rotated all times. Here, “rotated at all times” means that, when the main motor M is rotating, the relay roller is in a rotating state at all times. Mechanically, a clutch mechanism is not provided in the path by way of which power is transmitted from the rotary shaft M1 of the main motor M to the rotary shaft 23A of the relay roller 23, and the teeth sections of the respective adjacent transmission gears remain in a meshed state at all times. In the case of the relay roller employed in the present aspect, the sheet feeding cassette C2 is provided with a custom designed relay roller 42, and the MP tray 17 is also provided with a custom designed relay roller 46.

Before transporting the sheet, which has been transported by way of the sheet feeding roller 21 and the relay roller 23, to the transfer roller 27, the registration rollers 25 are for correcting the attitude of the sheet to a proper attitude and, subsequently, transporting the sheet to the transfer roller 27. Although the registration rollers 25 usually remain in a rotating state, the registration rollers 25 are temporarily suspended when the sheet has approached the registration rollers 25 (the approach is detected by a second detection sensor S2 which will be described later). As a result of the edge of the sheet coming into collision with the suspended registration rollers 25, the attitude of the sheet is corrected. Subsequently, the registration rollers 25 are again driven. Thus, the sheet is delivered to the transfer roller 27. As mentioned above, the registration rollers 25 also need to be stopped when necessary, as in the case of the sheet feeding roller 21. Therefore, an electromagnetic clutch mechanism 60B (corresponding to a second actuator of the present invention) is provided specifically for use with the registration rollers 25 (see FIG. 3).

The clutch mechanism 60B for use with the registration rollers is essentially identical to the clutch mechanism 60A for use with a sheet feeding roller. The clutch mechanism 60B is formed chiefly from the transmission gear G6, which has along an outer periphery thereof a lock receiving section, a lock arm (not shown) and a solenoid switch 65B. The clutch mechanism 60B differs from the clutch mechanism 60A for use with a sheet feeding roller in that the clutch mechanism 60B is set such that transmission of power is disconnected (interrupted) by means of supplying an excitation current and in that the transmission gear G6 is formed from a differential gear and is not provided with a notch section.

A state where supply of an excitation current to the solenoid switch 65B of the clutch mechanism 60B has been completed corresponds to the state of “completion of operation of the second actuator” in the present invention.

As shown in FIG. 1, a photosensitive drum 28 is disposed opposite the transfer roller 27. A scanner section 31, which can radiate a laser beam in order to form an electrostatic latent image on the photosensitive drum 28, is placed at a position above the photosensitive drum 28. Moreover, a development roller 33 and a toner storage section 35 are arranged side by side in front of (at right positions in FIG. 1) of the photosensitive drum 28. The photosensitive drum 28, the scanner section 31, the development roller 33, and the toner housing section 35 constitutes an image forming section and exhibit a function of forming a toner image on a sheet.

A press roller 37 is placed opposite the heating roller 38. The heating roller 38 has an elemental metal tube acting as a cylindrical member, and a halogen lamp is incorporated in the heating roller along the axial direction thereof. The surface of the heating roller 38 is heated to a fixing temperature by means of the halogen lamp. The press roller 37 is formed by means of coating the surface of a metal roller shaft with a rubber material, and the press roller 37is rotated so as to follow rotation of the heating roller 38 while being elastically pressed against the heating roller 38. The fixing unit 36 for thermally fixing a toner image on a sheet is formed from the press roller 37 and the heating roller 38. The transfer roller 27 and the heating roller 38 are rollers which are rotated at all times.

The sheet discharge roller 39 is located at a boundary that acts as a partition between the inside of the main body casing 11 and the sheet discharge section 14, and the sheet discharge roller 39acts to discharge the sheet, which has been transported by way of the fixing unit 36 and on which an image has been formed, to the sheet discharge section 14.

Three detection sensors S1 to S3 are provided in a path from the sheet feeding roller 21 to the sheet discharge roller 39. These three detection sensors S1 to S3 detect a transported state of the sheet, and all the three detection sensors perform detection operation by utilization of the same detection principle. Specifically, each of the detection sensors is formed from a photoelectric sensor consisting of a light emitting element and a light receiving element, which are disposed opposite each other to pair up each other and a pivotal member, which is a counterpart of the photoelectric sensor.

Reference numeral 81 in FIG. 4 designates a photoelectric sensor for use as the first detection sensor S1, 82 designates a photoelectric sensor for use as the second detection sensor S2, and 83 designates a photoelectric sensor for use as the third detection sensor S3. Reference numeral 85 designates a pivotal member for use with the first detection sensor S1, 86 designates a pivotal member for use with the second detection sensor S2, and 87 designates a pivotal member for use with the third detection sensor S3.

Each of the pivotal members 85 to 87 is pivotable about a hinge. The pivotal member 85 has a light blocking section for the photoelectric sensor 81 and a protruding section which protrudes into the sheet transport path. The pivotal member 86 has a light blocking section for the photoelectric sensor 82 and a protruding section that protrudes into the sheet transport path. The pivotal member 87 has a light blocking section for the photoelectric sensor 83 and a protruding section that protrudes into the sheet transport path. As the transported sheet travels across the protruding sections, the pivotal members 85 to 87 are pivotally displaced to thus change their attitudes. Thereby, the light blocking sections enter the detection areas of the photoelectric sensors to thus block the projected light, or recede from the detection areas to thus release optical paths. Thus, a change arises in the level at which detection light is received by the light receiving element, and hence passage of the sheet can be detected on the basis of the change in the receiving level of the light.

The detection functions of the respective detection sensors are sequentially described. First, the first detection sensor S1 is located between the sheet feeding roller 21 and the relay roller 23. The first detection sensor S1 detects completion of feeding of the sheet performed by the sheet feeding roller 21. Specifically, when the leading edge of the sheet collides against the pivotal member 85 of the first detection sensor S1, output of a detection signal is commenced. When the trailing edge of the sheet passes by the pivotal member 85, the output of the detection signal is stopped. Hence, the time at which the sheet feeding roller 21 has finished feeding a sheet can be detected at a point in time ts1 when the detection signal falls (see FIG. 7).

Next will be described the second detection sensor (corresponding to a detection sensor for detecting an approach of a sheet of the present invention) S2. This second detection sensor is placed at a position close to the registration rollers 25 between the relay roller 23 and the registration rollers 25. The second detection sensor S2 detects an approach of a sheet to the registration rollers 25. Specifically, when the leading end of the sheet reaches the second detection sensor S2 to thus pivotally displace the pivotal member 86, an output of the detection signal is commenced. Hence, an approach of a sheet can be detected at a point in time ts2 when the signal rises (see FIG. 7).

The third detection sensor S3 is placed between the registration rollers 25 and the transfer roller 27. Besides, the third detection sensor S3 is placed at a position close to the registration rollers 25. This third detection sensor S3 is for detecting an approach of a sheet to the transfer roller 27.

Reference numeral S4 in FIG. 4 designates a fourth detection sensor which acts in the same manner as does the first detection sensor S1. Specifically, the fourth sensor detects that the sheet feeding roller 41 has completed feeding a sheet from the sheet feeding cassette C2.

The electrical configuration of a laser printer will now be described.

As shown in FIG. 5, the laser printer 10 includes various circuit boards such as a main board 100, a front relay board 150, a rear relay board 160 and an additional tray board 170. A main CPU 120, RAM 121, ROM 122 and three types of timers 123A to 123C, each of which is formed from a counter circuit, are mounted on the main board 100. Further, the main motor M1, a polygon motor M2, a laser light source, the first detection sensor S1, the second detection sensor S2, and the third detection sensor S3 are electrically connected to the main board 100.

The main board 100 is provided with an interface 129, and the main board 100 is communicable with a higher level device (e.g., an unillustrated personal computer or the like) by way of the interface 129. Upon receipt of print data and a print command by way of the interface 129, the main board 100 controls the entirety of the laser printer 10, and the main board 100 causes the laser printer 10 to form a desired image on a sheet.

The front relay board 150 is provided in the front section of the laser printer 10, and the rear relay board 160 is provided in the rear section of the same. The solenoid switch 65A for use with the sheet feeding cassette c1, the solenoid switch 65B for use with the registration rollers 25, and the solenoid switch 65C for use with the MP tray 17 are electrically connected to the front relay board 150. A fixing thermistor, a DX unit sensor, and a sheet discharge sensor are electrically connected to the rear relay board 160. The solenoid switch 65D for use with the sheet feeding cassette C2 and the fourth detection sensor S4 are electrically connected to the additional tray board 170.

The relay boards 150, 160 and the additional tray board 170 are respectively connected with the main board 100 in a communicable manner by means of a signal line. The relay boards 150, 160 and the additional tray board 170 control respective connected devices in accordance with a command from the main board 100.

An ASIC (Application Specific Integrated Circuit) 130 is provided on the main board 100. The ASIC 130 has a CPU (corresponding to a control unit of the present invention) 131 and memory (e.g., nonvolatile memory such as flash memory or EEPROM and corresponds to a storage unit of the present invention) 132. The ASIC 130 is connected with the main CPU 120 in a communicable manner, and the ASIC 130chiefly controls sheet transport timing. Namely, operation start timings when the respective solenoid switches 65A to 65D are operated. A program storage area and a sheet transport data storage area are provided in the memory 132. A processing program for executing a control flow to be described below is written into the program storage area.

Subsequently, control of sheet transport timing performed by the ASIC 130 will now be described by reference to FIGS. 6 through 8. FIG. 6 is a flowchart of a processing program performed by the ASIC.

When processing is started, the ASIC 130 awaits a print command as indicated by step 10, and the ASIC 130enters an idle state. When a PC; i.e., a higher level device, has issued a print command, the print command is transmitted to the ASIC 130 by way of the interface 129 and the main CPU 120 of the main board 100. Thereby, processing proceeds to step 20. In the idle state, the sheet transport data storage area in the memory 132 remains in an initialized state. In the following descriptions, the sheet of the sheet feeding cassette C1 is selected by the higher level device as an object of printing.

In step 20, a control signal (corresponding to a sheet feeding command of the present invention) is sent to the solenoid switch 65A for use with the sheet feeding cassette C1 along the path formed from the ASIC 130 and the front relay board 150. Thereby, an excitation current is supplied from the power source to the solenoid switch 65A for use with the sheet feeding cassette C1 for only a given period of time t1 (see FIG. 7). The solenoid switch 65A is thus activated, and the power transmission mechanism is brought into a connected state. As a result, power is transmitted from the main motor M to the sheet feeding roller 21, whereby rotation of the sheet feeding roller 21 and transport of a sheet are commenced.

Subsequently, processing in the ASIC 130 proceeds to step 30, where a determination is made as to whether or not the transported sheet has approached the registration rollers 25. Once transport of the sheet has been started, time processing (processing for detecting a transported status of a sheet), which will be provided below, is performed by the main CPU 120. Moreover, determination processing pertaining to step 30 and steps 60 to 80 is performed by use of data pertaining to time processing. Therefore, time processing will be described prior to processing pertaining to step 30.

When transport of a sheet is commenced, the main CPU 120 counts three preset times ta, tb, and tc, which will be described below, from a predetermined reference time by use of the timers 123A to 123C (see FIG. 7). Respective reference times and a count completion time are stored in specified addresses R in the sheet transport data storage area in the memory 132.

More specifically, the rotation start time ts0 of the sheet feeding roller 21 is taken as a first reference time. The timer 123A counts a time for only the present time ta while taking the first reference time as a reference. Simultaneously with acquisition of the first reference time ts0 and the time when counting of the preset time ta has been completed, these times are written into respective specified addresses R1, R2.

The preset time (hereinafter also called “operation interval time”) ta is previously set by the main CPU (corresponding to an operation interval determination unit of the present invention) such that an interval between transport of sheets does not become shorter; namely, the number of operations of the solenoid switch 65A per unit time, which operates in association with transport of a sheet, does not exceed a preset number. The preset time is used in determination processing pertaining to step 70 which will be described later.

A time ts1 when the detection signal output from the first detection sensor S1 falls is taken as a second reference time. The timer 123B counts a time for only the preset time tb while taking the second reference time as a reference time. Simultaneously with acquisition of the second reference time ts1 and the time when counting of the preset time tb has been completed, these times are written into respective specified addresses R3, R4. The reason why the preset time tb is counted is that the preset time is used in determination processing pertaining to step 60, which will be described later.

A time ts2 when the detection signal output from the second detection sensor S2 rises; i.e., a time when an approach of a sheet to the registration rollers 25, is taken as a third reference time. The timer 123C counts a time for only the preset time tc while taking the third reference time as a reference time. Simultaneously with acquisition of the third reference time ts2 and the time when counting of the preset time tc has been completed, these times are stored into respective specified addresses R5, R6. The reason why the preset time tc is counted is that the preset time is used in determination processing pertaining to step 80 which will be described later.

When processing proceeds to processing pertaining to step 30, the ASIC 130 accesses the specified address R5 in the sheet transport data storage area in order to determine whether or not the transported sheet has approached the registration rollers 25. If the third reference time (a time when an approach of a sheet has been detected) ts2 is stored, the sheet is determined to have approached the registration rollers, and processing proceeds to step 40.

In step 40, the control signal is delivered to the solenoid switch 65B for use with the registration rollers 25 by way of the ASIC 130 and the front relay board 150. Thereby, the excitation current is supplied from the power source to the solenoid switch 65B, whereupon the solenoid switch 65B is activated. The power transmission mechanism, which has remained in a power transmittable connected state, is disconnected, whereby the registration rollers 25 are temporarily stopped. At this time, the leading end of the transported sheet collides against the registration rollers 25. As a result, even when the sheet has been transported from the sheet feeding roller 21 in an inclined position, the attitude of the sheet is properly corrected by the registration rollers.

As shown in FIG. 7, supply of the excitation current to the solenoid switch 65B for use with the registration rollers 25 is maintained for only the period of time t2 (corresponding to a predetermined time since operation of the present invention was started). Therefore, after elapse of the predetermined period of time, the power transmission mechanism is restored to a power transmittable connected state. Hence, the registration rollers 25 resume rotation. Therefore, the registration rollers 25, which have resumed rotation, resume the temporarily suspended transport of the sheet.

Subsequent to operation of the solenoid switch 65B, processing proceeds to step 50 by means of processing pertaining to the ASIC 130. In this step, a determination is made as to whether or not a next print job exists. When no next print job exists, No is selected through determination. Processing pertaining to control of the solenoid switches 65A to 65D, which are under control of the ASIC 130, is thereby completed. A case where the next job exists will be described later.

In the meantime, the sheet remains at any position along the sheet transport path L1 at this point in time, and the main CPU 120 performs print processing subsequent to this point in time. Specifically, when the leading edge of the sheet is detected by the third detection sensor S3, the main CPU 120 causes the image forming section to start print processing. Thereby, the photosensitive drum 28 is exposed to a high speed scan of a laser beam emitted from the scanner section 31. As the toner of the development roller 33 adheres to the surface of the photosensitive drum 28, a toner image is formed on the surface of the drum by means of negative development.

Subsequently, the sheet is transported between the photosensitive drum 28 and the transfer roller 27, whereby the toner image carried on the surface of the photosensitive drum 28 is transferred to the sheet. After having undergone a fixing process, the sheet is discharged to the sheet discharge section 14.

There will now be described a case where Yes is selected through determination in step S50; namely, where the next print job exists.

In step 60, a determination is made as to whether or not counting of the preset time tb has been completed. When processing proceeds to step 60, the ASIC 130 accesses the specified address R4 of the memory 132, to thus read the stored data from the address R4. At this time, when the timer 123B has not yet finished counting a time, no data will be written into the address R4. To prevent this, the timer 123B is determined to be still performing counting operation (No is selected through determination). When the timer 123B has finished counting the time and the time—at which the counting operation has been completed—is written into the address R4, the preset time tb is determined to have elapsed (Yes is selected through determination), and processing proceeds to step 70.

The reason why elapse of the preset time tb is determined is to set an appropriate interval between the trailing edge of the sheet that has already been sent and the leading edge of the sheet that will be sent now (the first condition).

In step 70, a determination is made as to whether or not the preset time ta has elapsed. To this end, when processing has proceeded to step 70, the ASIC 130 accesses the specified address R2 of the memory 132, to thus read the stored data. When the timer 123A has not yet finished counting the time, the time at which the counting operation has been completed is written into the specified address R2, whereby the preset time ta is determined to have elapsed (Yes is selected through determination), and processing proceeds to step 80.

The reason why elapse of the preset time ta; i.e., an operation interval time, is determined is to prevent the solenoid switch 65A from becoming heated and to seek stable operation of the clutch mechanism, and by extension, stable operation of the sheet transport system. Namely, the preset time (operation interval time) ta is an interval of supply of an excitation current that must be ensured to prevent the solenoid switch 65A (including the solenoid switches 65C, 65D) from becoming heated. If a sheet is supplied without adoption of such an operation interval time, the interval—at which sheets are transported—becomes excessively short, which in turn renders the interval—at which the excitation current is supplied to the solenoid switch—shorter (i.e., an average current value becomes greater), which heats the solenoid switch. In the present aspect, the operation interval time ta is set to about two seconds (the second condition).

By means of processing pertaining to step 70, the operation stated in the present aspect “an operation start timing of the first actuator is determined such that operation of the first actuator for the next sheet is started after elapse of the operation interval time since the start of operation of the first actuator for one sheet” is implemented.

In step 80, a determination is made as to whether or not the preset time tc has elapsed. When processing proceeds to step 80, the ASIC 130 accesses the specified address R6 of the memory 132, to thus read the stored data. When the timer 123C has not yet completed counting a time, the timer is determined to be performing counting operation (No is selected through determination). However, when the timer 123C has finished counting a time and the time when the counting operation has been completed is written into the specified address R6, the preset time tc is determined to have elapsed (Yes is selected through determination) and processing proceeds to step 90.

The reason why elapse of the preset time tc is determined is to ascertain that operation of the solenoid switch 65B for use with the registration rollers 25 with regard to the sheet has already been sent (i.e., a preceding job), i.e., supply of the excitation current to the solenoid switch 65B, has been completed (the third condition).

Completion of supply of the excitation current can be ascertained by means of counting the preset time tc. The reason for this is that counting of the preset time tc is performed simultaneously with initiation of the excitation current for the solenoid switch 65B (the third reference time ts2) and that the duration of the preset time tc is set so as to become longer than the time t2 during which supply of the excitation current is maintained (see FIG. 7). As above, the third reference time ts2 corresponds to the operation start time of the clutch mechanism 60B, in which supply of the excitation current to the solenoid switch 65B has been initiated. The time at which counting of the preset time ts has been completed corresponds to the state where operation of the clutch mechanism 60b, in which supply of power to the solenoid switch 65B is disconnected, has been completed. Hence, access is made to the specified addresses R5, R6 of the sheet transport data storage area in the memory 132, to thus ascertain the state of supply of the current to the solenoid switch 65B of the clutch mechanism 60B (corresponding to “operating status of the second actuator” of the present invention). By means of processing pertaining to step 80, “operation start timing of the first actuator is determined such that operation of the first actuator, which is to be started to transport the next sheet after transport of one sheet, is started after completion of operation of the second actuator for the one sheet” in the present invention is implemented.

In step 90, respective sets of data pertaining to the transported state of the sheet in the memory 132, namely, all the data stored in the specified addresses R1 to R6, are erased (initialized). Subsequently, processing proceeds to step 20. A control signal (corresponding to the “sheet feeding command” of the present invention) is sent to the solenoid switch 65A for use with the sheet feeding cassette C1 by way of the ASIC 130 and the front relay board 150. Thus, the excitation current is supplied to the solenoid switch 65A for use with the sheet feeding cassette C1 from the power source for only a given period of time t1. In subsequent processes, the main CPU 120 and the ASIC 130 perform processing in accordance with the previously described procedures. Thus, transport of the next sheet is started, and a desired image is formed.

Through processing pertaining to step 90, “the data pertaining to the operating status of the second actuator with regard to one sheet, which are stored in the storage unit, are deleted before the first actuator starts operation in order to transport the next sheet.”

The above aspect has been described by taking a case, where a sheet is supplied from the sheet feeding cassette C1, as an example. Processing is performed even in the case where a sheet is fed from the sheet feeding cassette C2 and the case where the sheet is fed from the MP tray 17, as in the case of the sheet feeding cassette C1. Thus, transport timing of a sheet is determined, and printing is also performed on the basis of this timing.

As mentioned above, according to the present aspect, three conditions are imposed on initiation of the next sheet after transport of one sheet. The first condition is that transmission of the previously sent sheet (one sheet) has already been completed (step 60). The second condition is that the operation interval time ta has elapsed since commencement of transport of the preceding sheet (step 70). The third condition is that the solenoid switch 65B for use with the registration rollers 25 has completed operation with regard to the previously sent sheet (step 80). Transport of a sheet is allowed only when all of these conditions have been fulfilled.

Thus, so long as the transport timing of the sheet is determined, delivery of superimposed sheets is avoided (an advantage of the first condition). Further, the operation interval time ta does not become excessively short. Hence, the mean value of the excitation current supplied to the solenoid switches 65A, 65B is maintained at an appropriate value, and hence the solenoid switches do not become heated (an advantage of the second condition).

Since transport of the next sheet is commenced after the clutch mechanism 60 has finished operation with regard to the previously sent sheet, simultaneous supply of an excitation current to both the solenoids 65A, 65B from the power source is avoided. Namely, load exceeding the performance of the power source can be avoided, and hence stable supply of power from the power source is achieved, so that operation of the laser printer 10 becomes stable (an advantage of the third condition).

The advantage of the third condition will be further described. The image forming apparatus of the present aspect is provided with the sheet feeding cassettes C1, C2 and the MP tray 17. Further, the sheet transport paths L1 to L3 differ from each other in terms of the length of the transport path from the sheet feeding start position to the registration rollers 25. When the sheet feeding start time ts0 is taken as a reference, the time—which elapses from when an approach of the sheet to the registration rollers 25 is detected until when supply of the excitation current to the solenoid switch 65B for use with the registration rollers is started—changes according to the sheet transport paths L1 to L3. FIG. 9 shows a timing chart achieved in a case where a sheet is transported from the sheet feeding cassette c2. In this case, when compared with the case where a sheet is transported from the sheet feeding cassette c1, the time ts2—when an approach of the sheet to the registration rollers 25 is detected—is seen to be delayed.

In consideration of this point, completion of operation of the solenoid switch 65B for use with the registration rollers is taken as a condition for transporting a sheet. Supply of an excitation current to the solenoid-switch 65A for use with a sheet feeding roller is always performed after completion of supply of the excitation current to the solenoid switch 65B for use with the registration rollers, regardless of the sheet transport paths L1 to L3.

The image forming apparatus of the present aspect controls operations of these solenoid switches 65A to 65D (control of sheet transport timings) by use of the custom designed ASIC 130 other than the main CPU 120 of the main board 100. As mentioned above, so long as operation is controlled by means of the custom designed ASIC 130, operation control (control of sheet transport timing) can be incorporated into an existing laser printer which does not control operation, without involvement of significant modifications to the printer.

According to the present aspect, the relay roller 23 is interposed between the sheet feeding roller 21 and the registration rollers 25 in each of the sheet transport paths L1, L2. When the sheet feeding roller 21 has started transporting operation, the sheet reaches the registration rollers 25 without being stopped by action of the relay roller 23. By means of such a configuration, the time which elapses from when transport of a sheet is started until when the sheet is subjected to image forming operation becomes shorter, and hence formation of an image can be performed efficiently. When compared with control performed in a case where the sheet is stopped during transport, control becomes simpler.

In addition, according to the present aspect, supply of power to the rollers 21, 23, 25, 27, 28, 37, 38, and 39 is implemented by a single drive source; namely, the main motor M. Hence, a smaller number of components is required.

The present invention has been described by reference to the descriptions and drawings. However, for instance, the following aspect is included in the technical scope of the present invention. Moreover, in addition to the following modifications, the present invention can be implemented while being modified in various manners within the scope of the invention.

In the present aspect, the object to be controlled by the ASIC 130 is set to the solenoid switch 65B for use with the registration rollers 25 and the solenoid switch 65A (including 65C, 65D) for use with the sheet feeding roller 21. Operation timings are controlled such that the solenoid switches 65A, 65B of the rollers 21, 25 are not operated simultaneously. However, the object of control is not limited to these, and various objects of control can be applied.

The present aspect has been described on the condition that sheets are sequentially sent from a single sheet feeding cassette. For instance, however, a plurality of sheets may be transported a long different sheet transport paths such that, after a sheet has been transported from the sheet feeding cassette C1, the next sheet is transported from the sheet feeding cassette C2 or the MP tray 17.

In such a case, a sheet transport timing is determined in accordance with the procedures of the present aspect (step 60 to step 80). Subsequently, when the solenoid switch is again activated in step 20, it is better to selectively switch the solenoid switches.

As the above, when sheets are sequentially transmitted from the different sheet feeding cassettes C1, C2, and MP17, sheets of different sizes are set in the respective cassettes, and the sheets of different sizes may be sequentially transported.

In the above aspect, the sheet feeding cassettes C1, C2 are constructed in two layers. However, the sheet feeding cassettes are not limited to this configuration. The sheet feeding cassettes may be formed into three or more layers.

In the present aspect, the times when the respective timers 123A, 123B, and 123C have completed counting operations are stored in the sheet transport data storage area in the memory 132. However, a bit flag (1 is set for completion of operation, and 0 is set for other cases) maybe employed, so long as completion of operation can be ascertained.

In the present aspect, supply of the excitation current to the solenoid switch 65C is determined to have been completed by means of the timer 123C counting the preset time tc. However, completion of supply of the excitation current may be detected directly. For instance, a custom designed circuit may be incorporated into the supply circuit that supplies an excitation current to the solenoid switch 65C, to thus detect completion of supply of the excitation current.

In the present aspect, the custom designed timers 123A, 123B, and 123C are provided for counting the preset times ta, tb, and tc. However, any timers are applicable, so long as they can count a time. For instance, the timers may be a loop timer embodied by software. When the loop timer is used, it is desirable to cause a CPU other than the CPU 131 of the ASIC 130 to perform counting operation. 

1. An image forming apparatus comprising: a sheet transport mechanism including: a sheet feeding unit that feeds a sheet from a sheet feeding cassette; an attitude correction unit that corrects an attitude of the sheet fed from the sheet feeding unit; and a drive source that drives the sheet feeding unit and the attitude correction unit; a first actuator that operates upon receipt of supply of power from a power source, when a sheet feeding command is issued, and transmits power of the drive source to the sheet feeding unit to commence driving of the sheet feeding unit; a second actuator that operates upon receipt of power from the power source and controls the attitude correction unit into an attitude correctable state, when a transported sheet has approached the attitude correction unit; and a control unit that determines an operation start timing of the first actuator, operation of the first actuator, which transports the next sheet after transport of one sheet, starting after operation of the second actuator for the one sheet has been completed.
 2. The image forming apparatus according to claim 1, further comprising: a storage unit that stores an operation status of the second actuator, wherein the control unit determines whether or not operation of the second actuator has been completed based on the operation status of the second actuator.
 3. The image forming apparatus according to claim 2, further comprising: a detection sensor that detects an approach of a sheet to the attitude correction unit, wherein operation of the second actuator is controlled so as to stop after elapse of a predetermined period of time since start of operation; the storage unit stores a time when the detection sensor detects an approach of a sheet; and the control unit uses the time when the detection sensor detects an approach of a sheet as a time when the second actuator is to start operation, and the control unit determines whether or not operation of the second actuator has been completed based on elapse of the predetermined period of time since the operation start time of the second actuator stored in the storage unit.
 4. The image forming apparatus according to claim 2, wherein data pertaining to an operating status of the second actuator with regard to one sheet are erased before the first actuator starts operation to transport the next sheet; and when operation of the second actuator is again started in association with transport of the next sheet, an operating status of the second actuator is stored in the storage unit.
 5. The image forming apparatus according to claim 1, further comprising a relay roller that is rotated at all times, wherein: the relay roller is interposed between the sheet feeding unit and the attitude correction unit; and when transport of a sheet from the sheet feeding unit has once been started, the sheet is transported up to the attitude correction unit without being stopped in a sheet transport path.
 6. The image forming apparatus according to claim 1, further comprising a transmission gear that transmits power, wherein: the sheet feeding unit and the attitude correction unit are connected to the drive source by the transmission gear; and the sheet feeding unit and the attitude correction unit rotate upon receipt of a supply of driving force from the drive source.
 7. The image forming apparatus according to claim 1, further comprising: an operation interval determination unit that determines an operation interval time of the first actuator such that the number of operations of the first actuator per unit time does not exceed a preset number, wherein the control unit determines the operation start timing of the first actuator to start operation of the first actuator with regard to the next sheet after operation of the second actuator for the one sheet has been completed and the operation interval time has elapsed since the start of operation of the first actuator for the one sheet.
 8. An image forming apparatus comprising: a sheet transport mechanism including: a sheet feeding unit that feeds a sheet from a sheet feeding cassette; an attitude correction unit that corrects an attitude of the sheet fed from the sheet feeding unit; and a drive source that drives the sheet feeding unit and the attitude correction unit; a first actuator that operates upon receipt of supply of power from a power source every time a sheet feeding command is issued and transmits power of the drive source to the sheet feeding unit to commence driving of the sheet feeding unit; a second actuator that operates upon receipt of power from the power source, when a transported sheet has approached the attitude correction unit, to control the attitude correction unit into an attitude correctable state; and an operation interval determination unit that determining an operation interval time of the first actuator such that the number of operations of the first actuator per unit time does not exceed a preset number; and a control unit that determines operation start timing of the first actuator, operation of the first actuator for the next sheet starting after the operation interval time has elapsed since the start of operation of the first actuator for one sheet.
 9. The image forming apparatus according to claim 1, wherein the sheet feeding unit is a sheet feeding roller; the attitude correction unit is registration rollers; and the drive source is a main motor.
 10. The image forming apparatus according to claim 9, wherein roller shafts of the sheet feeding roller and the registration rollers are connected to the main motor by a transmission gear; and the sheet feeding roller and the registration rollers rotate upon receipt of a supply of driving force from the main motor. 